1. Field of the Invention
The present invention relates to electrode pads for a Group III nitride compound semiconductor having p-type conduction, a Group III nitride compound semiconductor device having this type of electrode pads, and methods of forming these electrode pads. In particular, the invention relates to electrode pads for a Group III nitride compound semiconductor having p-type conduction with improved adhesive strength and luminous efficiency. Further, the invention relates to a method for forming the electrode pads and preventing the side etching of a protection film when forming a window through the protection film by etching.
2. Description of Background Information
Conventionally, gold (Au) deposited on a surface of a p-type conductive gallium nitride (GaN) layer is used as an electrode. Because the adhesive strength between the Au layer and the GaN layer is weak, an electrode layer made of Au peels easily away from the GaN layer during alloying processes performed on the GaN layer.
As a means to improve the adhesive strength, an electrode with a double layer structure is employed interposing a nickel (Ni) layer between the Au electrode layer and the GaN layer.
The electrode made of the Ni layer and the Au layer formed thereon also functions as an electrode pad for wire bonding. As shown in FIG. 5A, the semiconductor device is uniformly covered with a protection layer 20 to protect the surface of the device. As shown in FIGS. 5A and 5B, mask layers 21 are formed exposing some part of the protection layer 20 over the Au layer. The exposed portion of the protection layer 20 is removed by etching to form a window 20a. A lead wire is bounded to the electrode pad of the window 20a. 
However, because the adhesive strength between the Au layer and the protection layer 20 is weak, etching liquid penetrates between them. Thus, some part of the protection layer 20, even a part which is under the mask layers 21 are removed as shown in FIG. 5B. As a result, forming the window 20a as planned is difficult.
An object of the present invention is, therefore, to obtain an electrode pad strongly adhered to respective the Group III nitride compound semiconductor and the protection film.
Another object of the present invention is to improve ohmic characteristic of the electrode layer. The ohmic characteristic is defined as figure of volt-ampere (VI) characteristic. Improved ohmic characteristic includes fine linearity of VI characteristic and small contact resistance.
Further object of the present invention is to improve adhesive strength between the electrode layer and the Group III nitride compound semiconductor.
Further object of the present invention is to improve luminous intensity achieved by improvement in effective current density.
The Group III nitride compound semiconductor satisfies the formula AlxGayIn1xe2x88x92xxe2x88x92yN, wherein 0xe2x89xa6xxe2x89xa61, 0xe2x89xa6yxe2x89xa61, and 0xe2x89xa6x+yxe2x89xa61.
A first aspect of the present invention is directed to an electrode pad for a Group III nitride compound semiconductor having a p-type conduction. The electrode pad successively includes a first metal layer formed on one of the semiconductor layer and an electrode layer, a second metal layer formed on the first metal layer, and a third metal layer formed on the second metal layer. A protection film is formed covering over the surface of the device and exposing a central portion of the third metal layer. The second metal layer is made of gold (Au). A composite element of the first metal layer has an ionization potential lower than gold (Au), and a composite element of the third metal layer has adhesiveness to the protection film stronger than gold (Au).
The composite element of the first metal layer should preferably be at least one of nickel (Ni), Iron (Fe), copper (Cu), chromium (Cr), tantalum (Ta), vanadium (V), manganese (Mn), aluminum (Al), and silver (Ag). The composite element of the third metal layer should preferably be at least one of aluminum (Al), nickel (Ni), and titanium (Ti). Further, the protection film should preferably be made of silicon oxide eg., SiO, SiO2, and Si2OS3 or silicon nitride e.g., SiN2 and Si3N4. The most preferable combination of the composite metals for the electrode pad is the first metal layer comprising nickel (Ni), the second metal layer comprising gold (Au), and the third metal layer comprising aluminum (Al).
The electrode pad with the above identified structure may be formed directly on the semiconductor layer or on the electrode layer formed on the semiconductor layer depending on purposes. An LED, for example, requires an electric current to flow through a semiconductor layer vertically and uniformly for increasing an emission area. In this case, an electrode layer formed on the semiconductor layer is fairly wider than the electrode pad.
In order to improve an ohmic characteristic including lowering contact resistance, and Increase the adhesive strength between the electrode layer and the semiconductor layer, the electrode layer should preferably have a multi-layer structure at least comprising a first electrode layer formed on the semiconductor layer and a second electrode layer formed on the first electrode layer. The first electrode layer comprises an element that has an ionization potential that is lower than that of the second electrode layer and the second electrode layer comprises an element which has an ohmic characteristic to the semiconductor layer better than that of the first electrode layer. Heat treatment for alloying process of the semiconductor makes the element of the second electrode layer distributed more deeply into the semiconductor layer than that of the first electrode layer. Namely, the order of distribution of the elements is reversed by heat treatment. Before heat treatment, the element of the second electrode layer exists on that of the first electrode layer. After heat treatment, however, the element of the second electrode layer exists below that of the first electrode layer. The discovery of this phenomena is another aspect of the present invention.
The reversal of element distribution, however, does not occur at some portion of the electrode layer which is covered with an electrode pad. Accordingly, the portion of the electrode layer under the electrode pad has a poor ohmic characteristic i.e., a high contact resistance, and thus, flow of electric current detours around this portion This structure contributes to an improved luminous efficiency. Since emitted light under non-transparent electrode pad cannot be emitted outside, utilizing this structure enables the electric current to flow to effective portions of a device so that emitted light can extend outside. Consequently, enlarging effective current density improves luminous efficiency. In view of the improved luminous efficiency, the protection film is not necessary and the electrode pad may be formed In a double or a single layer structure.
The first electrode layer should preferably be made of at least one of nickel (Ni), iron (Fe), copper (Cu), chromium (Cr), tantalum (Ta), vanadium (V), manganese (Mn), aluminum (Al), and silver (Ag), and the second electrode layer be at least one of palladium (Pd), gold (Au), iridium (Ir), and platinum (Pt). The most preferable combination of the composite metals of the electrode layer is the first electrode layer comprising nickel (Ni) and the second electrode layer comprising gold (AU). In this case, heat treatment reverses the relative positions of Ni and Au. Gold (Au) moves deeper with respect to the semiconductor layer than nickel (Ni). Heat treatment may preferably be carried out at a temperature generally ranging from 400xc2x0 C. to 700xc2x0 C.
As another aspect of the present invention, the Group III nitride compound semiconductor device using the above described electrode pads includes one of a light-emitting diode (LED), a laser diode (LD), and a transistor.
Because the composite metal of the first metal layer has an ionization potential that is lower than that of a second metal electrode layer, a strong adhesive strength of the metal layer to the semiconductor can be maintained. A protection film formed on the third metal layer covering the sides of the first, second, and third metal layers has a strong adhesive strength to the third metal layer. Accordingly, etching the inner sides of the protection film can be prevented during an etching process to remove sore portion of the protection film. As a result, the protection film can function as intended.
Further, the electrode pad shields the effect of heat treatment to the layers thereunder. The portion of the electrode layer under the electrode pad does not undergo a reversal in its distribution of elements. Accordingly, the contact resistance of some portion of the electrode layer which is under the electrode pad is high, and thus, electric current detours this portion and flows into other parts of the electrode layer. Consequently, luminous efficiency of the device is improved by increasing effective current density for emission.
The other portion of the electrode layer which is not covered with the electrode pad undergoes a reversal in its distribution of elements. Because the element of the first electrode layer has an ionization potential lower than that of the second electrode layer, the first electrode layer has a greater adhesive strength than the second electrode layer. The second electrode layer, which has a work function larger than that of the first electrode layer. That is, the second electrode layer has a higher ionization potential than that of the first electrode layer, and has a better ohmic characteristic to the semiconductor layer. Providing a heat treatment on the electrode layer causes the element of the first electrode layer to move to the surface of the second electrode layer. In particular, the element of the second electrode layer penetrates through the first electrode layer and significantly into the semiconductor layer. A larger portion of the element of the first electrode layer moves and distributes on the surface of the electrode layer. This reversal of the distribution of the elements of the first and the second electrode layers improves both the adhesive strength and the ohmic characteristic between the electrode layer and the semiconductor layer. Particularly, when nickel (Ni) is used as the composite material of the first electrode layer and gold (Au) as that of the second electrode layer, Au penetrates through the Ni first electrode layer. Thus, the ohmic characteristic is improved, i.e., contact resistance is lowered, by using gold (Au) and the adhesive strength between the electrode layer to the semiconductor layer is maintained by using nickel (Ni). On the other hand, some portion of the electrode layer which is under the electrode pad has a high contact resistance, but has enough adhesive strength because Ni remains on the surface of the semiconductor layer.
A Group III nitride compound semiconductor device having an electrode structure as described above improves its device characteristics such as reduction of both applied voltage and contact resistivity and improvement in its reliability. In addition, when the electrode described above is used for an LED and an LD, emission efficiency is improved.
The above and other objects, features, advantages, and characteristics of the present invention are further described in the following detailed description with reference to the accompanying drawings by way of non-limiting exemplary embodiments of the present invention, wherein like reference numerals represent similar parts of the present invention throughout the several views.